The all-weather and night capability of infrared radar and imaging systems has led to their increasing use and to the development of lens systems suitable for use with them. One such type of system is known by the acronym "FLIR" from "forward looking infrared". These systems are preferably operated in the 2 to 20 micron wavelength region of the spectrum. Such systems are normally equipped with a minimum number of lens elements since the relatively high transmission loss in lenses having a spectral bandpass in the infrared precludes the use of a large number of lenses for aberration correction as is common practice in the visible region particularly in zoom systems. This transmission loss limitation thus poses an even greater problem than is encountered in the visible region when one attempts to provide such a FLIR system with a zoom lens capability. This is particularly true of such a zoom system which is of the bolt on or afocal type the output of which is a collimated beam of energy focused at infinity and which can provide not only acceptably small transmission loss but also usefully high image quality when used with a prime lens.
Most of the previously known zoom systems, even in the visible wavelength region, are of the finite focus or image forming output type rather than of the afocal or attachment type intended for use with a prime focusing lens. U.S. patents typical of this image forming type at visible wavelengths are: U.S. Pat. No. 3,377,119 to Takano; US. Pat. No. 3,433,559 to Vockenhuber et al; U.S. Pat. No. 3,454,321 to Klein; and U.S. Pat. No. 3,597,048 to Bertele. Focusing lens systems intended for use at infrared wavelengths are shown in U.S. Pat. Nos. 3,439,969 to Kirkpatrick and 3,825,315 to Altman et al.
In addition to their classification as either image forming or afocal in type, zoom lens systems may also be classified as being either of the optically compensated or mechanically compensated type and may be further classified as being of the Galilean or non-Galilean type. These distinctions are per se well known in the art as may be seen, for example, by reference to a book entitled "Modern Optical Engineering" by Warren J. Smith published in 1966 by McGraw-Hill Company. The author states at page 243 that a "mechanically compensated" zoom system is one in which the defocusing (which is inherent by virtue of the motion of one of the movable lens elements) is eliminated by introducing a nonlinear compensating motion of one of the other adjacent lens elements of the system which motion is effected by a cam arrangement to provide the non-linearity. At page 244 it is noted that the other technique for reducing the focus shift in a variable system is called optical compensation wherein two or more alternate (rather than adjacent) lenses are linked and moved together with respect to the fixed lens between them.
The further distinction of types of telescopic systems as being astronomical (or inverting), terrestrial (or erecting), or Galilean is discussed by the author beginning at page 209. At page 210 the author points out that in the Galilean type the internal image of the system is never actually formed. In the Galilean type the object for the eye lens is a "virtual" rather than a "real" object, no inversion occurs, and the final image presented to the eye is erect and unreversed. Since there is no real image formed in a Galilean telescope there is no location where cross hairs or a reticle may be inserted. Hence, for purposes of this specification, a non-Galilean lens system will be defined as one in which an internal real image is formed within the system, that is, somewhere between the first and last lenses of the system along the optical axis.
With these distinctions in mind it may be noted that even with respect to afocal visible wavelength zoom systems the prior art has depended primarily on the use of Galilean type systems having no real intermediate image. Such systems are illustrated by U.S. Pat. Nos. 3,320,014 to Macher and 3,679,292 to Motoaki. Where an effort has been made to use a non-Galilean system having a real intermediate image, an optically compensated system such as shown in U.S. Pat. No. 3,438,689 to Wehr has been necessarily used.
It is an object of this invention to provide a mechanically compensated non-Galilean afocal zoom lens attachment of variable magnification having as few lenses as possible for use with a prime imaging lens of fixed focal length.
It is a more specific object of the present invention to provide such a system suitable for use in the infrared wavelength region of the spectrum which system overcomes the above noted problems and limitations inherent in the design of an optical system intended for use at such infrared wavelengths and which is capable of producing high image quality throughout the zoom range of the afocal system.